Hybrid electric vehicles include both an engine and at least one electric machine (motor/generator) to provide drive torque to the wheels. There are various types of hybrid electric vehicles. For example, a “parallel” hybrid vehicle typically includes clutches that selectively enable either or both the engine and the motor to provide drive torque. A “series” hybrid vehicle typically includes an electric motor that is always driveably connected to the road wheels, and an engine that is not mechanically connected to the wheels. In other words, the engine does not provide any of the torque required to propel the vehicle. Rather, in a series hybrid vehicle, the engine powers a generator to produce electrical energy that is stored in the battery and/or used by the motor. A “series-parallel” hybrid takes advantage of both series and parallel hybrids. A clutch separates the engine from the electric machine, which may be arranged along a common dimensional axis. This allows the electric machine to still propel the vehicle alone, but also allows the engine to be connected and propel the vehicle and, optionally, charge the electric machine while propelling the vehicle.
A series-parallel hybrid, for example, is provided with the benefit of being capable of operating in a plurality of drive modes. In an electric mode, the electric machine alone propels the vehicle. In an engine mode, the engine alone propels the vehicle. In a hybrid mode, both power sources propel the vehicle. In a regeneration mode, the engine can be disconnected form the electric machine or, if connected, arranged to consume as little or no fuel as possible. If it is connected, the engine will decelerate the vehicle by converting the mechanical movement into losses, while the electric machine converts mechanical movement into electric energy to be stored in the battery.
In such a hybrid vehicle, there is a shift schedule for the electric mode, a shift schedule for hybrid mode, and a shift schedule for regeneration mode. Each shift schedule attempts to place the electric machine, engine, or both in the most efficient or best power-operating point for the respective drive mode. During a tip-out shift in the transmission, the vehicle state may move from electric mode to hybrid mode, or from electric mode to regeneration mode, or from hybrid mode to electric mode due to some other event not related to a driver action (such as the state-of-charge of the battery being depleted or being full). Simultaneous engine starting/stopping or transmission shifting may reduce drivability.